1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly, to an organic dye-containing optical recording medium capable of writing once.
2. Description of the Related Art
An optical recording medium for reading out only has been put to practical use as a compact disc (hereinafter referred to as CD). This medium can be produced by forming recording information and a prepit or a pregroove for tracking to reproduce the recording information on a substrate made of a transparent polycarbonate and the like using a press or the like in advance, forming a reflective layer composed of a metal film such as Au, Al and the like on the surface where the pit is formed, forming a protective layer made of a photo-curable resin overlying the reflective layer, providing label-printing with a photo-curable ink on the protective layer. The CD has spread widely for the purpose of recording or reproducing music, image, data, program and the like. Specifications for recording and reproducing signals of the CD are prescribed by the CD standards, and a reproducing apparatus pursuant to the CD standards have spread widely as a CD player.
Recently, an optical recording medium capable of writing once (hereinafter referred to as CD-R) that the recorded disc meets the CD standards has attracted attention since the CD-R can be reproduced with a commercially available CD player.
The CD-R is an optical recording medium producible by forming on a transparent substrate an organic dye-containing recording layer, a metallic reflective layer and protective layer and is disclosed in Nikkei Electronics (No. 465, published on Jan. 23, 1989, page 107), Japanese Patent Application Laid-Open No. Hei 2-132657, Japanese Patent Application Laid-Open No. Hei 2-147286, Japanese Patent Application Laid-Open No. Hei 2-168448 and the like. The CD-R has been known as an optical recording medium capable of recording information as signals by changing an organic dye-containing recording layer using laser beam irradiation from a transparent substrate side. It is possible to reproduce the recording and reproducing signals by a CD player since a metallic reflective layer results in a high reflectivity and a clear contrast. For the purpose of preventing a dye-containing recording layer and a metallic reflective layer from a scratch, a protective layer is formed on the reflective layer and moreover label-printing is provided on the protective layer to indicate disc specifications and the like. The label-printing is not only for decoration but also plays a role to show recording contents, disc specifications and the like and is vital for practical use. As far as a protective layer and a print portion of an optical recording medium are concerned, Japanese Patent Application Laid-Open No. Hei 2-7249 and the like disclose that both a protective layer and a print portion are formed by screen-printing to simplify a manufacturing method. Japanese Patent Application Laid-Open No. Hei 3-40239 discloses that it is possible to improve mechanical properties and durability of a protective layer by forming a first print portion and a second print portion on the protective layer. Although these prior arts show some characteristics concerning a production process or layer constitution, conditions of a print portion and a protective layer including the effects of the print portion are not disclosed therein. It is considered that this is attributed to that it is not necessary to pay attention thereto since a print portion does not affect recording characteristics of a medium such as a conventional CD, a write-once medium, an optomagnetic recording medium and the like.
With respect to conditions of a protective layer employed for a CD-R, for example, Japanese Patent Application Laid-Open No. Hei 3-232130, Japanese Patent Application Laid-Open No. Hei 3-232131, Japanese Patent Application Laid-Open No. Hei 3-232132 and the like define hardness, expansion coefficient and shrinkage rate of a protective layer are defined. However, conditions of a print portion and a protective layer including the effects of the print portion are not shown therein.
The present inventors have investigated a CD-R having constitution that an organic dye-containing recording layer, a metallic reflective layer, a protective layer and a print portion are formed successively on a transparent substrate and capable of obtaining recording and reproducing signals satisfying the CD standards. As a result, it has been found that in spite of existence of the protective layer, the print portion surprisingly affects characteristics of the medium especially after a weathering test which is required for the CD standards such as a heat-cycle test, a high temperature and high humidity test and the like. It is considered that this is a phenomenon characteristic of the CD-R medium attributable to a poor adhesion of the interface between an organic dye and a metal.
In order to improve scratch resistance of the surface of a CD-R medium, it has been attempted to form two layers or more as a protective layer and provide an antiscratch layer as an outermost layer. In this case, characteristics of the medium are deteriorated remarkably due to a print portion.